1. Field of the Invention
The present invention relates to an electrostatic powder coating method and apparatus which are suitably used for top coat of automobiles and decorative coating of steel-made furniture or the like.
2. Description of the Related Art
An electrostatic powder coating apparatus of prior art includes a powder supply unit and a spray gun. In the powder supply unit of prior art, as shown in FIG. 10, a fluid-bed type hopper 1 is partitioned by a porous resin plate 21 into an upper fluidizing container 1a and a lower air chamber 1b. The air chamber 1b is provided in its side with a fluidizing air supply port 3 through which compressed air A is supplied to the air chamber 1b and then to the fluidizing container 1a through the porous resin plate 21. With supply of the compressed air, powder 23 in the fluidizing container 1a is evenly mixed with the air, sucked from the fluidizing container 1a through an injector 9 for feeding to the spray gun 101, and then sprayed toward a product to be coated along with a flow of conveying air from a nozzle opening at the gun tip.
During a spraying process, a DC high voltage is applied to a pin type discharge electrode provided at the gun tip, so that monopolar ions are generated by corona discharge in the vicinity of the electrode.
When the powder passes by the electrode, powder particles are given with electric charges upon impingement against the ions. The charged powder particles are deposited by electrostatic forces on the product to be coated which is held at the grounded potential.
Then, the powder layer deposited on the product to be coated is finished into a coating film through the steps of melting and hardening performed in an atmosphere at high temperatures of about 150 to 250.degree. C. Powders for use in electrostatic powder coating generally have an average particle size of 30 to 40 .mu.m for easiness in handling.
The prior art powder supply unit described above have, however, problems below.
(1) Particularly for powders accompanying a difficulty in fluidization, such as fine particle powder or powder having large specific gravity, the powder and air are not well mixed with each other even if the amount of air is simply increased, and passages 24 allowing the air to pass therethrough are formed in a powder layer L. Therefore, most of the supplied air straightly blows through the powder layer L and, hence, the powder is not fluidized.
In that case, a fluidization accelerant is required to be added to the powder for improving fluidity thereof. However, such an improvement in property of the powder pushes up the cost of powder production. Further, the improvement in fluidity may cause drawbacks such as a reduction in charging ability and a deterioration in appearance of coating films.
(2) On the other hand, when a large amount of compressed air is used as fluidizing air, much of powder is expelled out of the powder container along with sprayed air and the rate of utilization of the powder is lowered. In the case of fine particle powder, particularly, the method of supplying a large amount of compressed air is not preferable because the amount of powder expelled out of the fluid-bed type hopper is increased.
(3) Of powders accompanying a difficulty in fluidization, fine particle powder is apt to easily aggregate above all and is hard to handle. Since fine particle powder has a larger surface area per unit mass than other usual powders, aggregation thereof is accelerated for the following factors:
a: aggregation based on electrostatic attractions between powder particles, b: aggregation by softening of powder particles at high temperatures, and c: aggregation through adsorbed water at high humidity.
Therefore, if powder in the fluid-bed type hopper is always fluidized by a large amount of compressed air, the friction between powder particles or between powder particles and the fluid-bed type hopper causes changes in physical properties of the powder, e.g., aggregation thereof, and an appearance of the coating film after the steps of coating and baking is adversely affected.
Also, using high-temperature or high-humidity compressed air as fluidizing air tends to easily cause changes in physical properties of powder, e.g., aggregation thereof, and hence adversely affect an appearance of the coating film after the steps of coating and baking as with the above case.
(4) The powder aggregated in the fluid-bed type hopper and the powder adhering in a powder feeding line extending from the supply unit to the spray gun are responsible for spitting and adversely affects an appearance of the coating film.
The spray gun of prior art has problems below.
As mentioned above, powders having an average particle size of 30 to 40 .mu.m are generally used in electrostatic powder coating, but smoothness of coating films formed of such powders is fairly inferior to that formed by solvent based coating. For this reason, powder coating has not been widely employed in the field where decorative coating films having good smoothness is required, e.g., in top coat of automobile bodies and steel-made furniture.
In the electrostatic powder coating method, smoothness of a coating film can be improved by, e.g., thickening the coating film, improving fluidity of powders, or making up powders of fine particles.
However, thickening the coating film gives rise to problems of an increase in the coating cost due to the increased amount of powder used and a deterioration in appearance of the coating film due to generation of electrostatic repulsion.
In order that powders having an average particle size of 30 to 40 .mu.m provide smoothness comparable to that obtainable by solvent based coating without thickening the coating film, it is thought to improve fluidity of powders. But this method causes the coating film to easily sag during the baking step, and eventually deteriorates an appearance of the coating film.
On the other hand, if fine particle powders having an average particle size not greater than 25 .mu.m, preferably in the range of 5 to 20 .mu.m, are used, smoothness of the coating film is improved, thinning of the coating film is enabled, and a reduction in the coating cost is also expected.
However, easiness in handling fine particle powders is considerably worsened as follows, making it very difficult to handle fine particle powders by commercially available electrostatic powder coating apparatus.
Even if such powder can be coated, satisfactory properties of the coating film are not obtained in points of evenness and appearance of the coating film because of a difficulty in stable supply of the powder and aggregation of the powder.
(1) The powder having a small particle size is strongly affected by electrostatic forces, and adhesion forces of fine particles are increased. As a result, the fine particles of the powder are apt to aggregate together or adhere to a powder supply tube, a powder spray gun and so on. Therefore, the powder having a small particle size accompanies a difficulty in stable supply to the spray gun, and the coating film cannot have an even thickness.
(2) The aggregated powder or the powder peeled off from the powder supply tube and the spray gun adheres to the product surface to be coated in the form of lumps, which considerably mar an appearance of the coating film.
(3) As the particle size of the powder is reduced, the transfer efficiency of powder onto the product to be coated is greatly affected by the amount of air sprayed from the spray gun.
Thus, the powder reaching the product to be coated is blown away in a considerably amount by the flow of conveying air and, eventually, the transfer efficiency is lowered.
As explained above, fine particle powders are more strongly affected by aerodynamic forces of powder conveying air and electrostatic forces. One simple method of increasing the transfer efficiency by the use of existing coating apparatus is to reduce the amount of powder sprayed per gun. Specifically, with a reduction in the amount of powder sprayed, the amount of conveying air is reduced and the charging rate is increased, which results in the improved transfer efficiency. But, if the amount of powder sprayed per gun is reduced, the number of spray guns must be increased because the amount of powder to be deposited for coating a product is unchanged. This is not preferable from the viewpoint of equipment cost.
Furthermore, in multilayer coating, there occur other problems such as a reduction in the transfer efficiency and generation of electrostatic repulsion by the coating film. A description will be made below of the case of coating automobile bodies as a typical example of multilayer coating.
A coating film for automobile bodies is formed of a multilayer coating film in three or four layers comprising: an electro-coat, an primer surfacer, and a top coat (a combination of base coating and clear top coating) which are laminated successively in this order.
Those layers except the electro-coat and the metallic base coating can be formed by powder coating. But since coating films one over another is subjected to greater electrostatic limitations than the case of coating a film directly on a metal-made product, there arise problems such as a reduction in the transfer efficiency and texture roughing of the coating film caused by electrostatic repulsion.
For example, when electrostatic coating is made on a product having an under coat already formed thereon by applying a high voltage of about -80 kV to the spray gun, the surface of the under coat of the product to be coated traps electric charges of ions flying from the spray gun and, hence, the surface charge density of the product to be coated is raised.
Therefore, the surface potential of the product to be coated becomes as high as minus several kV, causing electrostatic repulsion between charged powder particles sprayed toward the product to be coated from the spray gun and the product to be coated. This reduces the transfer efficiency. Electrostatic repulsion is also caused between the powders deposited on the product to be coated. As a result, a disadvantage in appearance of the coating film, i.e., roughing of the film texture, is more likely to occur.
In view of the state of art as set forth above, an object of the present invention is to realize fluidization of powder that is difficult to fluidize, and supply such powder to a spray gun stably. Another object of the present invention is to coat fine particle powder evenly on the surface of a product to be coated without causing aggregation of the powder, and form a coating film having good smoothness. Still another object of the present invention is to coat a film on a product, which has an under coat already formed thereon, with high transfer efficiency, and form a coating film which is free from texture roughness and has good smoothness.